Bending stress operates on a holder when a milling tool such as a face mill and an endmill, particularly, a milling tool used with a large number of long cutting edges is used in a cutting. The holder has had specific stiffness for the purpose of preventing such bending stress from causing great flexure. In an actual cutting, however, bending stress operating on the holder changes in size in accordance with a cutting condition. Accordingly, lack of stiffness of the holder causes great flexure. This results in a rise of chatter vibration in the cutting, in some cases. Particularly in a heavy cutting in which a depth of cut is large, great increase in cutting resistance easily causes chatter vibration. In view of the above, widely used as means for reducing the cutting resistance in the cutting or the like has been a combination of plural cutting inserts having a flank face including a main cutting edge, the flank face being divided by a groove part.
As such a cutting insert, known has been a cutting insert in which a rake face continued from a main cutting edge includes a positive rake angle and a groove part is used for dividing a flank face including the main cutting edge (Japanese Unexamined Patent Publication JP-A 7-299636 (1995), for example). In the cutting insert, a chip formed by the main cutting edge is divided into pieces having a small thickness by the groove part, so that the cutting resistance is reduced while a biting property for a workpiece becomes excellent. This allows an effect of suppressing the chatter vibration in the cutting to be achieved.
In a cutting or the like in which a depth of cut is particularly large, however, even a cutting insert having the above-described structure is not always sufficient for suppressing the chatter vibration since the cutting resistance increases significantly.
It can be considered as one of measures for further suppressing the chatter vibration in the cutting or the like that the number of groove parts for dividing the flank face including the main cutting edge is made large to reduce the cutting resistance. In this case, however, there is a problem that the strength of a divided cutting edge part is greatly reduced, and thereby, a fracture of the cutting insert is easily caused.